In the 19th century Kowalevsky realized that ascidians must be close relatives of the vertebrates because their eggs develop into a tadpole larva – i.e., a larval form that shares essential, and homologous, features with embryos of vertebrates like frogs.  The tadpole has a bulbous head (although for solitary ascidian tadpoles, ‘head’ is somewhat optimistic, as the only sensory organs it contains at this stage are an ocellus and an otolith) from which a muscular tail extends, posterior to the blastopore (=anus).  This tail is largely what reveals vertebrate affinities, because it is supported by a perfectly good notochord and includes a dorsal nervous system which developed by the in-rolling of a plate of ectoderm. 

This video shows the transition from neurula – the stage in which the neural tube is formed – through development of the notochord and tail.  In the gastrula and neurula, the presumptive notochord cells are found in an “anterior” position.  The ascidian notochord consists of just 40 cells, as compared to the hundreds or thousands in a vertebrate embryo, but as in vertebrate embryos these cells migrate amongst each other, converging medially after gastrulation to extend the rudimentary notochord.  The nub of the tail emerges as the notochord rudiment extends posteriorly, eventually adopting a ‘stack-of-coins’ arrangement at the core of the embryonic axis. 

As the notochord lengthens, other cells differentiate to form the spinal cord, tail muscles and tail epidermis.  Having formed a one-cell-wide stack, notochord cells develop large gel-filled vacuoles and secrete an extracellular sheath.  The notochord presumably behaves as a skeletal support for the tail, which tadpoles, lacking cilia, must thrash violently to swim.  At metamorphosis, ascidian tadpoles generally resorb the tail, dissolving the notochord.  In most vertebrates, too, the notochord is a transient structure: ours becomes our intervertebral discs.

— text by Katie Bennett & George von Dassow